1. Field of the Invention
The invention relates to apparatus, in particular a microscope, for the examination of a number of gemstones, pearls or similar small objects, comprising means for creating dark-field illumination and a slide-type displaceable specimen holder of plastics material which has an elongate recess in its upper surface.
2. Description of the Prior Art
It is known to inspect gemstones or crystals using a magnifying lens or a microscope. The illumination is created either according to the transmitted-light principle or the incident-light principle. In the first case the light passes through the transparent or translucent object, while in the second case the object is illuminated from the side and the reflected light rays are gathered. The object being examined can be observed against a light field or against a dark field in the case of illumination according to either of the aforesaid principles. In the case of light-field illumination the background appears bright and the object dark, while with dark-field illumination the object appears bright and the background appears dark.
For the examination of gemstones it is also known to use microscopes (commonly microscopes having nipples as object holders) equipped with polarization optics, i.e. with a polarizer and an analyzer. In order better to be able to observe light refraction, reflection and total internal reflection at the facets of gemstones, in many cases transmitted-light, dark-field illumination is used, in which the object is illuminated from the side or the central zone of the illuminating light beam is screened out.
For the examination of particular objects, such as for example uncut gemstones, which are rough and without defined crystal faces, it is necessary to inspect these objects in an immersion liquid. The refraction and total internal reflection phenomena arising from the different refractive indices of air and specimen can in this way be at least greatly reduced, depending upon the actual refractive indices, thus producing a picture which is closer to reality. However, with this method of examination, damage to or flaws in a gemstone are not outwardly visible. For the examination of a series of features of gemstones, such as for example double refraction, perhaps observation of double-refractive inclusions, as well as stresses, the use of polarized light is indispensable. With crossed polarizers, double-refracting crystals appear bright, while single-refracting crystals remain dark (see also the article by Will Kleber, VEB Verlag Technik Berlin, "Einfuhrung in die Kristallographie", 10th Edition, 1969, pages 296 to 300).
The provision of vessels containing immersion liquid in the vertical light beam path of microscopes has proved to be disadvantageous for various reasons. First of all, small glass beakers of about 2cm depth have been used, which are positioned on the illuminating aperture of the microscope stage. Upon movement of this glass beaker it can happen that the immersion liquid, which is extraordinarily aggressive chemically, can spill over and lead to corrosion and damage to the optics. A further disadvantage of such immersion systems is that, as a result of movement of the liquid or of the liquid surface, one obtains a blurred image. For these reasons, for the examination of objects in immersion liquid, microscope systems have been developed in which the light beam travels horizontally through the immersion liquid. In this so called "German" system movement of the liquid surface does not impair the examination process.
For the displaying of large batches of gemstones, particularly for the dimensional analysis of gemstones of average quality, so called sorting boards of plastics material are used. The gemstones are arrayed on these sorting boards and the array is examined from above in order to observe the effect of the stones. Higher quality stones on the other hand are viewed from below, for example in order to observe the cut of the gem.
In U.S. Pat. No. 3,554,631 there is described a specimen holder, particularly for interference microscopes, which is provided with a groove which tapers down towards the base. The width of the groove at the upper surface is .congruent.100 .mu.m and at the bottom is approximately 10 .mu.m. The dimensions of the groove are so chosen that the interference rings can be observed individually over the whole width of the groove, with the width of the groove at the upper surface of the specimen holder preferably being smaller than the diameter of the object field. This permits the interference rings to remain fully visible during the whole measuring process and to have approximately the same spacing from each other at the bottom of the groove.
In one microscope described in DE-GM 1 958 962 there is used a system having dark-field illumination and a slide-type displaceable specimen holder of transparent plastics material. Below the specimen holder is positioned an incandescent lamp, the light from which travels either directly to the specimen holder or first to a reflector laterally surrounding the lamp and thereafter to the specimen holder. Between the incandescent lamp and the specimen holder there is positioned a diaphragm. If the diaphragm is open, then the light rays can strike the specimen holder directly and perpendicular to its surface, while if the diaphragm is closed the light rays from the reflector strike the specimen holder at an angle. Additionally, this known microscope comprises a fluorescent lamp for the incident-light illumination, in order to improve the visibility of mineral inclusions in gemstones for example. Such incident-light illumination also makes it possible to inspect diamonds.
The specimen holder of this known microscope is formed as a displaceable rail with indentations, in order to be able to examine a number of gemstones in succession. For the displacement of the rail it is proposed to provide a manually actuated knurled wheel, with this wheel preferably being positioned outside the field of view. This leads however to an elongation of the means used to hold the specimen holder in place. For a stepless displacement of the specimen holder it is preferred to use a friction drive which can consist for example of a rubber roller, a belt pulley, a belt and a knurled wheel.
In its upper surface the specimen holder is provided with the aforesaid recess which as a rule consists of two channels (indentations) having a V-shaped cross-section. The side faces of the channels preferably are each set at 45.degree., so that they together form an angle of 90.degree.. The gemstones are arranged at the beginning of the examination process with the known microscope in one of the two channels and are moved, after appropriate sorting, into the other free channel. For the observation of the gemstones the diaphragm is closed, so that only indirect rays strike the specimen holder from below, with the result that only the channels are lighted. This means that only the stones are struck by the rays and the remaining surfaces appear dark.
However, the known microscope described above has considerable disadvantages in practical use. The incandescent lamp used for illumination gives rise on the one hand to a considerable heating of the specimen holder and on the other hand the luminosity which it produces is not satisfactory, even with the use of a 60 watt lamp. For this latter reason the inclusion of a polarization system is not possible. The specimen holder is heated to more than 60.degree. C., even with the use of an electric bulb of about 30 watts. It is also a particular disadvantage that plastics materials such as Plexiglas for example are poor conductors of heat, so that the illuminated underside of the specimen holder is noticeably warmer than the upper side. This leads to a deformation or stressing of the specimen holder, with the result that its capacity to slide and be displaced are substantially impaired and, in some circumstances, possibly after several hours use of the system, it is no longer usable.
Microscopes in which a glass-fiber cable is used for the purpose of illumination are known. In one such microscope as described in U.S. Pat. No. 4,505,555 a glass body is positioned close to the end surface of a glass-fiber cable and serves to pick up the light transmitted by the glass-fiber cable and to transfer light to the output surface with a uniform light intensity distribution. The light beam which is thus created is then passed into the optical system of the microscope.
In the case of pearls, which are basically divided between natural pearls and cultured pearls, these look practically the same when viewed externally, but nevertheless have very different monetary values. In order to be able to distinguish between natural and cultured pearls, an instrument known as a lucidoscope has been developed for example, in which the pearl is immersed in an immersion solution, is irradiated by a strong light source, and is observed through a microscope. If the pearl under examination is a cultured pearl, then with suitable orientation of the pearl, striations of the mother-of-pearl core are visible. However, this effect only occurs occasionally, so that this instrument is not really suitable for the examination of cultured pearls, and especially not for such pearls which are covered by thick scale. Furthermore, it is disadvantageous that one has to have considerable screening due to the strong light source, and also that because of the need to use the immersion liquid one cannot examine strings of pearls or the like. Other methods of examining pearls have been based on the use of X-rays, X-ray refraction pattern processes and X-ray shadow image methods (see for example Schlossmacher, "Edelsteine und Perlen", Stuttgart, 1969).